Fill in the blank with the most appropriate word.
________ me with my suitcase, please?

In the given sentence "________ me with my suitcase, please?" the most appropriate word to fill in the blank is "Will you help?"

The sentence is a request or a plea for assistance, and the correct way to form such a request in English is by using the modal verb "will" to indicate politeness. The correct structure for this request is "Will you help me with my suitcase, please?"

The other options do not use the appropriate form for making a request. Option A, "Did you help," is in the simple past tense, which is not suitable for asking for help in a polite manner. Option B, "Are you going to help," is a question about future intentions, but it is not as polite as using "will" in this context. Option C, "Are you helping," is in the present continuous tense, which also does not convey the same level of politeness as using "will."

Therefore, the correct answer is "D. Will you help."